A subcatchment 
is a hydrologic unit that generates runoff and produces a hydrograph that can be routed downstream. It can have two parts, pervious and impervious areas. A pervious area is a surface where water infiltrates into the upper soil zone, but in an impervious area, there is no infiltration.
Regarding your project, you can divide a study area into different subcatchments. The outlet of a subcatchment can be a node of the drainage system or another subcatchment.
| Parameter Name | Description | Default Value |
|---|---|---|
| NHYD | Hydrograph number. | — |
| NAME | Name of the component. | — |
| System Type | Specifies the junction’s system type, whether it belongs in the overland, storm, or sanitary system. | Blank |
| Outlet | The NHYD of the outlet node. | — |
| COMMENT 1, 2, and 3 | Comment for the command. | — |
| Tag | Optional category or classification. | — |
| Area [ha] | Area of subcatchment. | 5 |
| *Width [m] | Width of the overland flow path (m or ft). | 500 |
| Slope [%] | Average surface slope (%). | 0.5 |
| Curb Length | Curb length (if needed for pollutant buildup functions). | 0 |
| Storm Index | The storm index of the storm to use with the rain gauge group. | 1 |
| % Imperv | Percent of impervious area (%). | 25 |
| N-Imper | Manning’s n for the impervious area. | 0.013 |
| N-Perv | Manning’s n for the pervious area. | 0.1 |
| Dstore-Imperv [mm] | Depth of depression storage on the impervious area (mm or in). | 1.5 |
| Dstore-Per [mm] | Depth of depression storage on the pervious area (mm or in). | 5 |
| %Zero-Imperv | Percent of the impervious area with no depression storage (%). | 25 |
| Subarea Routing | Choice of internal routing between pervious and impervious sub-area. | OUTLET |
| Percent Routed | Percent of runoff routed between sub-areas. | 100 |
| Infiltration | Infiltration method. | **HORTON |
| Groundwater | Groundwater flow parameters. | NO |
| Snow Pack | Name of snow pack parameter set (for snow melt analysis only). | — |
| LID Controls | LID controls | — |
| Land Uses | Assignment of land uses to subcatchment. | — |
| Initial Buildup | Initial pollutant buildup on subcatchment. | — |
*There are different methods to calculate the length and width of a basin. A simple way is the Equivalent Rectangle Method to determine the shape of the watershed.
L= (C√A+ √(C^2-1.2544A))/1.12
B= (C√A- √(C^2-1.2544A))/1.12
Which A is the area of the watershed (km^2). C is the Gravelius coefficient. L is the equivalent rectangular length (km). B is the equivalent rectangular width (km).
**There are five techniques to model the infiltration into the unsaturated soil zone in the previous area.
- Horton
- ModifiedHorton
- GreenAmpt
- ModifiedGreenAmpt
- Curve Number
